The present invention is generally directed to processes for the preparation of homopolymers of acrylic monomers and to copolymers containing homoacrylate segments. More specifically, the present invention relates to polymerization processes which provide homoacrylate and copolymeric acrylate resin products which possess narrow polydispersity properties and which polymerization processes proceed with high monomer to polymer conversion. In particular, this invention relates to acrylate polymerization processes which yield homoacrylate and copolymers containing homoacrylate segments having number average molecular weights (M.sub.n) above about 100 to about 1,000 and having a polydispersity ratio of the weight average molecular weight (M.sub.w) to the number average molecular weight (M.sub.n) of from about 1.0 to about 2.0.
The present invention provides in embodiments a pseudoliving polymerization process that enables the synthesis of narrow polydispersity homoacrylate and copolymeric acrylate resins from acrylate and acrylate derivative monomers. The process can, in embodiments, use known free radical initiators in combination with an oxygenated stable free radical agent and acrylate monomers to afford narrow polydispersity homoacrylate and copolymeric acrylate resins.
In other embodiments the acrylate polymerization processes of the present invention can be used to prepare block copolymers and multi-block polymer having narrow polydispersity properties wherein at least one of the blocks is optionally water soluble and subsequently added blocks or segments may be only partially or entirely water insoluble thereby providing a means for preparing surface active or surfactant materials having well defined polydispersity and hydrophobe-lipophobe balance (HLB) properties.
Many polymerization processes used for the synthesis of narrow polydispersity acrylate and related resins, such as anionic, cationic, and group transfer polymerization processes, are severely limited by the need for anhydrous reaction conditions and monomers which do not contain protic or reactive functional groups, for example, hydroxy (OH) carboxy (CO.sub.2 H), amino (NH), and the like. As a consequence, these processes are not readily applicable to the polymerizaton of acrylate monomers since these monomer materials tend to be hydroscopic and any associated water may readily destroy the polymerization initiator component, for example, the hydrolysis or protonation of organolithium reagents, or in other ways cause the polymerization to fail entirely or to be industrially inefficient.
Conventional free radical polymerization processes that are used to polymerize acrylate monomers inherently give broad polydispersity resin products or require that sophisticated processing conditions and materials handling protocols be employed.
The polymer resins produced by processes of the present invention, in embodiments, are essentially monomodal, that is the molecular weight distribution is narrow and indicative of a Poisson character and without shoulders or side bands. In embodiments, by repeating the heating step, comprising the combined initiation and polymerization step, there is provided a means for obtaining monomodal mixtures of polymer resins that are compositionally the same resin type having characteristics of both narrow polydispersity and known or selectable modality greater than 1. In embodiments, the process of the instant invention provides a means for conducting homoacrylate polymerization processes on multikilogram or larger scales. The aforementioned embodiments may be accomplished in a one or single pot reactor environment. In embodiments, polymeric chain growth proceeds by a pseudoliving mechanism and can provide resins of variable molecular weights from very low to very high, for example, less than about 2,000 up to about 200,000 while maintaining narrow molecular weight distributions or polydispersities. In embodiments, block and multi-block copolymers can be synthesized by the aforementioned stable free radical moderated homoacrylate polymerization processes wherein each block formed is well defined in length by the sequentially added and reacted monomer and wherein each additional block that is formed also possesses a narrow molecular weight distribution.
It is generally accepted that known anionic and cationic polymerization processes used for the preparation of narrow polydispersity resins, block and multiblock polymers are not believed possible in aqueous or protic solvent containing polymerization media, or the aforementioned protonic or reactive functional groups, reference the aforementioned copending application number U.S. Pat. No. 5,312,704. The present invention enables the preparation of homoacrylate containing block and multi-block copolymers which preparation was heretofore not achievable in stable free radical moderated, free radical initiated polymerization systems.
Of the known polymerization processes a preferred way to prepare polymers or copolymers having a narrow molecular weight distribution or polydispersity is by anionic processes. The use and availability of resins having narrow polydispersities in industrial applications is limited because anionic polymerization processes must be performed in the absence of atmospheric oxygen and moisture, require difficult to handle and hazardous initiator reagents, and consequently such polymerization processes are generally limited to small batch reactors. In addition, the monomers and solvents that are used must be of high purity and anhydrous thereby rendering the anionic process more expensive than alternatives which do not have these requirements. Thus, anionic polymerization processes are difficult and costly. It is desirable to have free radical polymerization process that provides narrow molecular weight distribution homoacrylate containing resins that overcomes the shortcomings and disadvantages of the aforementioned anionic polymerization processes.
Similarly, group transfer polymerization (GTP) processes have limitations and disadvantages, such as anhydrous reaction conditions and expensive reagents, which disadvantage GTP processes for large scale industrial applications.
Free radical polymerization processes are generally chemically less sensitive than anionic processes to impurities in the monomers or solvents typically used and are substantially or completely insensitive to water. There has been a long felt need for an economical free radical polymerization process which is suitable for preparing narrow polydispersity resins by aqueous processes.
Acrylate polymerization processes known in the art proceed by a free radical mechanism providing resins of broad polydispersities and generally high molecular weights. The present invention relates to homoacrylate and copolymeric acrylate polymerization processes that proceeds via a pseudoliving free radical mechanism and provides homoacrylate containing resins of high, intermediate, or low molecular weights and with narrow polydispersities. The present invention provides product resins with a latent thermally reactive functional group on at least one end which can be used for further reaction to prepare other resins with complex architectures. The present invention, in embodiments, provides polymerization processes that enable control of resin molecular weight, weight distribution, modality of the products, and the like properties.
Acrylate polymerization processes are industrially important, and are used for the synthesis of numerous copolymers, for example, deflocculating or dispersent polymers. However, resins prepared by known acrylate polymerization processes typically have broad polydispersities and high molecular weights. When low molecular weight resins were required, a chain transfer agent is typically added to limit the extent of chain growth by way of premature chain termination events and which agent is, for example, an unpleasant smelling thiol. Polymers prepared by thiol type chain transfer mediated polymerization processes are irreversibly terminated with functional groups, such as alkyl sulfides, which preclude further free radical reactions and therefore limits the utility of the polymer resin products produced therefrom.
The present invention is directed to pseudoliving homoacrylate polymerization processes which permit the economic preparation of homoacrylate and copolymeric acrylate containing narrow polydispersity resins with low, intermediate, or high molecular weights. The low molecular weight resins can be prepared without a chain transfer agent or molecular weight modifier which provides several advantages over conventional chain transfer mediated polymerization processes.
Copolymers prepared by free radical polymerization processes inherently have broad molecular weight distributions or polydispersities, generally greater than about four. One reason is that most free radical initiators selected have half lives that are relatively long, from several minutes to many hours, and thus the polymeric chains are not all initiated at the same time and which initiators provide growing chains of various lengths at any time during the polymerization process. Another reason is that the propagating chains in a free radical process can react with each other in processes known as coupling and disproportionation, both of which are chain terminating and polydispersity broadening reaction processes. In doing so, chains of varying lengths are terminated at different times during the reaction process which results in resins comprised of polymeric chains which vary widely in length from very small to very large and thus have broad polydispersities. If a free radical polymerization process is to be enabled for producing narrow molecular weight distributions, then all polymer chains must be initiated at about the same time and premature termination by coupling or disproportionation processes must be avoided or eliminated.
Contemporary environmental issues and pollution concerns are prompting greater use of certain biodegradable polymers, among these are water soluble polymers as described by F. Lo, J. Petchonka, J. Hanly, Chem. Eng. Prog., July, 1993, p. 55-58, the disclosure of which is incorporated by reference herein in its entirety. In embodiments of the present invention are provided water soluble and biodegradable polymeric resins.
In other applications, such as water treatment, it is particularly important that polymer products have a narrow molecular weight distribution, that is, low polydispersity. In conventional free radical polymerization processes, polydispersity rises rapidly as the high molecular weight fraction of the polymer mixture increases. In many processes designed to produce low molecular weight polymers, high molecular weight fractions are observed because there is insufficient control over chain-chain coupling and branching. These high molecular weight fractions tend to dominate the viscosity characteristics of the polymer product and can detract from polymer performance. Other processes designed to produce low molecular weight polymers result in the formation of excessive amounts of oligomeric products, for example, dimers and trimers, which can also detract from the polymer performance. These by-products do not have as much of an affect on the viscosity characteristics of the polymer mixture. However, they do affect the number average molecular weight such that it is no longer indicative of the properties of the polymer product.
In addition to chain-chain coupling and branching, processes for producing low molecular weight polymer products tend to have high polydispersities resulting from the methods used to reduce the residual monomer content of the polymer product. Methods of reducing the residual monomer content of the polymer mixture include post-polymerization processing which employs additional initiator, extended periods at elevated temperatures, and use of comonomeric scavengers. These methods tend to broaden the molecular weight distribution or polydispersity. Therefore, unless the polymer mixture has a sufficiently low polydispersity to begin with, the above mentioned methods used to reduce residual monomer content will raise polydispersity of the product to an unacceptable level.
Practitioners in the art have long sought an inexpensive, efficient and environmentally efficacious means for producing polymers having operator controllable or selectable molecular weight properties and further processes which selectively afford a wide variety of different polymer product types and have narrow molecular weight distribution properties.
In the aforementioned U.S. Pat. No. 5,322,912 there is disclosed free radical polymerization processes for the preparation of a thermoplastic resin or resins comprising: heating from about 100.degree. to about 160.degree. C. a mixture comprised of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form the thermoplastic resin or resins with a high monomer to polymer conversion and a narrow polydispersity. A broad spectrum of free radical reactive monomers, including acrylate and acrylic acid derivatives, are suitable for use in the highly versatile polymerization process. However, while the aforementioned acrylic monomers were readily incorporated into various copolymer resins, for example, alternating copoly(styrene-n-butyl acrylate), it was not possible to prepare homopolymeric acrylate containing resins. Although not desired to be limited by theory, it is believed that the inability to form homoacrylate resins or polymeric segments was the result of a combination of factors including among others the irreversible termination of telomeric products, that is, initiator, monomer and stable free radical coupled products.
The following patents are of interest to the background of the present invention, the disclosures of which are incorporated by reference herein in their entirety:
In U.S. Pat. No. 5,268,437, to Holy, issued Dec. 7, 1993, discloses a high temperature aqueous processes for the polymerization of monoethylenically unsaturated carboxylic monomer to produce low molecular weight, water-soluble polymer products useful as detergent additives, scale inhibitors, dispersents and crystal growth modifies. Suitable monomers include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, crotonic acid, and itaconic acid. The reactions are run at temperatures ranging from about 130.degree. to 240.degree. C., preferably from about 140.degree. to about 230.degree. C., with polydispersities less than 2.5. The process can be continuous, semicontinuous, or batch.
In U.S. Pat. No. 4,546,160, to Brand et al., issued Oct. 8, 1985, is disclosed a process to continuously bulk polymerize acrylic monomers to prepare low molecular weight, uniform polymers employing minor amounts of initiator and, optionally solvents, at short residence times and moderate reaction temperatures to provide high yields of a product with polydispersities less than 3, suitable for high solids applications.
U.S. Patent 4,581,429 to Solomon et al., issued Apr. 8, 1986, discloses a free radical polymerization process which controls the growth of polymer chains to produce short chain or oligomeric homopolymers and copolymers including block and graft copolymers. The process employs an initiator having the formula (in part) .dbd.N--O--X, where X is a free radical species capable of polymerizing unsaturated monomers. The molecular weights of the polymer products obtained are generally from about 2,500 to 7,000 having polydispersities generally of about 1.4 to 1.8, at low monomer to polymer conversion. The reactions typically have low conversion rates and use relatively low reaction temperatures of less than about 100 degrees C., and use multiple stages.
U.S. Pat. No. 5,059,657 to Druliner et al., issued Oct. 22, 1991, discloses a polymerization process for acrylic and maleimide monomers by contacting the monomers with a diazotate, cyanate or hyponitrite, and N-chlorosuccinimide, N-bromosuccinimide or a diazonium salt. The polymer produced can initiate further polymerization, including use in block copolymer formation.
U.S. Pat. No. 4,736,004 to Scherer, Jr. et al., issued Apr. 5, 1988, discloses novel persistent perfluorinated free radicals which, upon thermal decomposition, yield free radicals which can be used to polymerize polymerizable monomers containing ethylenic unsaturation.
Other references cited in an international search report for the aforementioned Application Number U.S. Pat. No. 5,322,912 are: J. Am. Chem. Soc., 1983, 5706-5708; Macromol., 1987, 1473-1488; Macromol., 1991, 6572-6577; U.S. Pat. No. 4,628,019 to Suematsu et al., issued Aug. 10, 1986; U.S. Pat. No. 3,947,078 to Crystal, issued Aug. 10, 1976; and U.S. Pat. No. 3,965,021 to Clemens et al., issued Jun. 22, 1976.
One method of achieving control of polymer molecular weight is through the use of efficient chain transfer agents, but this approach has several drawbacks. This approach irreversibly incorporates the structure of the chain transfer agent into the polymer chain. This can be undesirable since that structure will have an increasing effect on the properties of the polymer as molecular weight decreases. Furthermore, the chain transfer agents commonly employed are mercaptans. These materials are expensive and have objectionable odors associated with their presence. Other common chain transfer agents are hypophosphites, bisulfites, halogenated hydrocarbons such as carbon tetrabromide, and alcohols. These also add to the cost of the process, introduce undesired functionally to the polymer, can introduce salts into the product, and may necessitate an additional product separation step to remove the chain transfer agent from the reaction mixture.
Another way of lowering the molecular weight of the polymers product is by increasing the amount of free radical initiator. This approach adds considerably to the cost of production and may result in polymer chain degradation, crosslinking, and high levels of unreacted initiator remaining in the product. In addition, high levels of initiator may also result in high levels of salt by-products in the polymer mixture which is known to be detrimental to polymer performance in many applications. The same is true for chain stopping agents such as sodium metabisulfite. Among the preferred free-radical initiators for aqueous polymerization is hydrogen peroxide. It is relatively inexpensive, has low toxicity, and does not produce detrimental salt by-products. However, hydrogen peroxide does not decompose efficiently at conventional polymerization temperatures, that is less than about 100.degree. C., and large amounts must be used to generate enough radicals to carry out a polymerization.
High levels of metal ions together with high levels of free radical initiator have also been tried as means for controlling molecular weight. This method is taught in U.S. Pat. No. 4,314,044 where the ratio of initiator to metal ion is from about 10:1 to about 150:1 and the initiator is present from about 0.5 to about 35 percent based on the total weight of the monomers. Such an approach is unsuitable for some products, such as water treatment polymers, which can not tolerate metal ion contaminants in the polymer product. In addition, the product is usually discolored due to the presence of the metal ions.
The following references are also of interest: U.S. Pat. Nos. 3,682,875; 3,879,360; 3,954,722; 4,201,848; 4,542,182; 4,581,429; 4,777,230; 5,059,657; 5,173,551; 5,191,008; 5,191,009; 5,194,496; 5,216,096; and 5,247,024.
In homoacrylate polymerization reaction processes of the prior art, various significant problems exist, for example difficulties in predicting or controlling both the polydispersity and modality of the polymers produced. These acrylate polymerization processes produce polymers with high weight average molecular weights (M.sub.w) and low number average molecular weights (M.sub.n) resulting in broad polydispersities or low molecular weight (M.sub.n) and in some instances low conversion. Further, acrylate polymerization processes of the prior art are prone to generating excessive quantities of heat since the polymerization reaction is exothermic. As the viscosity of the reaction medium increases dissipation of heat becomes more difficult. This is referred to as the Trommsdorff effect as discussed and illustrated in Principles of Polymerization, G. Odian, 2nd Ed., Wiley-Interscience, N.Y., 1981, page 272, the disclosure of which is entirely incorporated herein by reference. This is particularly the situation for reactions with high concentrations of soluble monomer, for example greater than 30 to 50 percent by weight soluble monomer, which are conducted in large scale reactors with limited surface area and limited heat dissipation capacity. Moreover, the exothermic nature of free radical acrylate polymerization processes is often a limitation that severely restricts the concentration of reactants or the reactor size upon scale up.
Further, gel body formation in conventional free radical polymerization processes may result in a broad molecular weight distributions and/or difficulties encountered during filtering, drying and manipulating the product resin, particularly for highly concentrated reactions.
These and other disadvantages are avoided, or minimized with the homoacrylate and copolymeric acrylate polymerization processes of the present invention.
Thus, there remains a need for homoacrylate acrylate polymerization processes for the preparation of narrow polydispersity polymeric resins containing homoacrylate segments by economical and scalable free radical polymerization techniques and which polymers retain many or all of their desirable physical properties, for example, hardness, low gel content, processability, clarity, high gloss durability, and the like, while avoiding the problems of gel formation, exotherms, volume limited and multi-stage reaction systems, purification, performance properties of the polymer resin products, and the like, associated with prior art free radical acrylate polymerization methodologies.
The homoacrylate and copolymeric acrylate polymerization processes and homoacrylate containing thermoplastic resin products of the present invention are useful in many applications, for example, as a variety of specialty applications including toner and liquid immersion development ink resins or ink additives used for electrophotographic imaging processes or where monomodal or mixtures of monomodal narrow molecular weight resins or block copolymers with narrow molecular weight distribution within each block component are suitable for use, for example, in thermoplastic films and aqueous or organic solvent borne coating technologies.